1. Field
Embodiments described herein relate generally to a nonvolatile memory device utilizing a variable resistor.
2. Description of the Related Art
Conventionally, flash memory, in which memory cells having a floating gate structure are NAND-connected or NOR-connected to configure a memory cell array, is widely known as an electrically rewritable nonvolatile memory. In addition, ferroelectric memory is known as a nonvolatile memory capable of high-speed random access.
On the other hand, resistance varying memory using a variable resistor as the memory cell is proposed as a technology aiming for further miniaturization of memory cells. Known variable resistors include, for example, phase change memory elements in which change in the degree of crystallinity/amorphousness in a chalcogenide compound is used to change the resistance value, MRAM elements employing resistance change due to the tunnel magnetoresistive effect, memory elements for polymeric ferroelectric RAM (PFRAM) in which the resistance element is formed by a conductive polymer, and ReRAM elements in which electric pulse application is used to cause resistance change (US2009/0046495).
This resistance varying memory has its memory cells configurable by a series circuit of a diode and a variable resistor in place of a transistor, and therefore has the advantage that stacking is easy and adoption of a three-dimensional structure allows an even higher degree of integration to be achieved (JP 2009-94344 A).
In case of that a silicon-based diode is adopted for the diode, deposition and impurity activation temperatures rise. Accordingly, heat load leads to difficulties in multiple stacking of the memory-forming layers. In addition, difficulty of film thinning in the silicon-based diode results in a rise in aspect during processing of the memory-forming layers, which is a main cause of worsening yield.
In contrast, a MIIM diode is formed by stacking two or more kinds of insulating films having different band gaps and permittivities, whereby low temperature formation of the insulating film is possible and film thinning is easy.
However, conventional resistance varying memory requires a forming operation in which energy from outside is used to form a current path in the variable resistor in its initial state. This forming operation causes the insulating film used in the MIIM diode to deteriorate, leading to generation of read disturb and program disturb. That is, there is a problem that operation of the resistance varying memory is not stably performed.